Latest research in football - week 9 - 2019

As previous literature updates, I have performed a PubCrawler search looking for football articles in NCBI Medline (PubMed) and GenBank databases.

Following studies were retrieved for this week:

#1 Does maturation influence neuromuscular performance and muscle damage after competitive match-play in youth male soccer players?
Reference: Eur J Sport Sci. 2019 Feb 18:1-10. doi: 10.1080/17461391.2019.1575913. [Epub ahead of print]
Authors: De Ste Croix M, Lehnert M, Maixnerova E, Zaatar A, Svoboda Z, Botek M, Varekova R, Stastny P
Summary: Poor neuromuscular control and fatigue have been proposed as a risk factor for non-contact injuries especially around peak height velocity (PHV). This study explored the effects of competitive soccer match-play on neuromuscular performance and muscle damage in male youth soccer players. 24 youth players aged 13-16y were split into a PHV group (-0.5 to 0.5y) and post PHV group (1.0-2.5y) based on maturity off-set. Leg stiffness, reactive strength index (RSI), muscle activation, creatine kinase (CK), and muscle soreness were determined pre and post a competitive soccer match. Paired t-tests were used to explore differences pre and post competitive match play and independent sample t-tests for between groups differences for all outcome measures. There was no significant fatigue-related change in absolute and relative leg stiffness or muscle activation in both groups, except for the gastrocnemius in the post PHV group. RSI, CK and perceived muscle soreness were significantly different after soccer match-play in both groups with small to large effects observed (ES:0.41-2.82). There were no significant differences between the groups pre match-play except for absolute and relative leg stiffness (P < 0.001; ES = 1.16 and 0.63 respectively). No significant differences were observed in the fatigue related responses to competitive match play between groups except for perceived muscle soreness. The influence of competitive match-play on neuromuscular function and muscle damage is similar in male youth around the time of PHV and those post-PHV indicating that other factors must contribute to the heightened injury risk around PHV.

#2 Spikes in acute:chronic workload ratio (ACWR) associated with a 5-7 times greater injury rate in English Premier League football players: a comprehensive 3-year study
Reference: Br J Sports Med. 2019 Feb 21. pii: bjsports-2018-099422. doi: 10.1136/bjsports-2018-099422. [Epub ahead of print]
Authors: Bowen L, Gross AS, Gimpel M, Bruce-Low S, Li FX
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Summary: We examined the relation between global positioning system (GPS)-derived workloads and injury in English Premier League football players (n=33) over three seasons. Workload and injury data were collected over three consecutive seasons. Cumulative (1-weekly, 2-weekly, 3-weekly and 4-weekly) loads in addition to acute:chronic workload ratios (ACWR) (acute workload (1-week workload)) divided by chronic workload (previous 4-week average acute workload) were classified into discrete ranges by z-scores. Relative risk (RR) for each range was then calculated between injured and non-injured players using specific GPS variables: total distance, low-intensity distance, high-speed running distance, sprint distance, accelerations and decelerations. The greatest non-contact injury risk was when the chronic exposure to decelerations was low (<1731) and the ACWR was >2.0 (RR=6.7). Non-contact injury risk was also 5-6 times higher for accelerations and low-intensity distance when the chronic workloads were categorised as low and the ACWR was >2.0 (RR=5.4-6.6), compared with ACWRs below this. When all chronic workloads were included, an ACWR >2.0 was associated with a significant but lesser injury risk for the same metrics, plus total distance (RR=3.7-3.9). We recommend that practitioners involved in planning training for performance and injury prevention monitor the ACWR, increase chronic exposure to load and avoid spikes that approach or exceed 2.0.

#3 Measurements and Coach Assessments for Efficient Talent Selection in Elite Youth Football Science or Coaches' Eye? - Both! Beneficial Collaboration of Multidimensional
Reference: J Sports Sci Med. 2019 Feb 11;18(1):32-43. eCollection 2019 Mar.
Authors: Sieghartsleitner R, Zuber C, Zibung M, Conzelmann A
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Summary: Due to the tremendous popularity of youth football, practitioners in this domain face the ongoing question of the most effective solutions in early talent selection. Although the scientific community has suggested multidimensional models for some time, coach assessments and motor performance tests remain common. Earlier research has determined the strengths and weaknesses within these different approaches. The current investigation directly compared the effectiveness of each approach in talent selection (coach assessment vs. motor performance tests vs. multidimensional data). A sample of 117 youth football players, their parents, and coaches participated in multidimensional measurements in the U14 age category (coach assessments, motor performance tests, psychological characteristics, familial support, training history, and biological maturation). The area under the curve (AUC [95% CI]) from receiver operating characteristic indicated the prognostic validity of each approach in predicting U19 player status five years after the assessments (professional vs. non-professional). Motor performance tests (0.71 [0.58; 0.84]) showed a lower AUC than the multidimensional data (0.85 [0.76; 0.94], p = 0.02), whilst coach assessments did not differ from the two others (.82 [.74; .90]). Further, combined talent selection approaches, especially the use of coach assessments and multidimensional data together, were significantly better at predicting U19 player status (0.93 [0.87; 0.98], p = 0.02 vs. multidimensional data only). Although certain limitations may impede further insights (summation of data, skipped use of non-linear statistics), scientific claims for using multidimensionality within talent selection were confirmed to be fruitful. In particular, the combination of the subjective coaches' eye with scientific data may buffer the mutual weaknesses of these different approaches. Future research should focus on optimizing the output of promising multidimensional models. Knowledge of detailed values relating to specific dimensions within these models and the implementation of enhanced non-linear statistics may enable further improvements in the field of talent selection.

#4 Playing football on artificial turf as a risk factor for fifth metatarsal stress fracture: a retrospective cohort study
Reference: BMJ Open. 2019 Feb 20;9(2):e022864. doi: 10.1136/bmjopen-2018-022864.
Authors: Miyamori T, Nagao M, Sawa R, Tumilty S, Yoshimura M, Saita Y, Ikeda H, Kaneko K
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Summary: The fifth metatarsal stress fracture is a common injury among football players. Although several risk factors have been proposed, the association between the playing surface and development of fifth metatarsal stress fractures (MT-5) has not been evaluated. We conducted an epidemiological study using a computer-based survey to investigate the association between the playing surface and development of MT-5. This study included 1854 football players, of which 41 experienced MT-5 within the past 24 months. Baseline demographic data and the percentage of time spent playing on artificial turf and clay fields were compared between the non-MT-5 and MT-5 player groups, and the risks for development of MT-5 associated with the playing surfaces were estimated by univariate and multivariate analyses. There were significant differences in body mass index, years of play, playing categories and playing time on artificial turf between non-MT-5 and MT-5 groups (p<0.05). Generalised estimating equations analyses adjusted for multiple confounders demonstrated that relative to the risk of playing <20% of the time on each surface, the OR (OR: 95% CI) for MT-5 for playing on artificial turf >80% of the time increased (3.44: 1.65 to 7.18), and for playing on a clay field 61%-80% of the time, the OR decreased (0.25: 0.11 to 0.59). A higher percentage of playing time on an artificial turf was a risk factor for developing MT-5 in football players. This finding could be beneficial for creating strategies to prevent MT-5.

#5 Physical Fitness Characteristics of High-level Youth Football Players: Influence of Playing Position
Reference: Sports (Basel). 2019 Feb 16;7(2). pii: E46. doi: 10.3390/sports7020046.
Authors: Bujnovky D, Maly T, Ford KR, Sugimoto D, Kunzmann E, Hank M, Zahalka F
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Summary: The aim of this study was to determine whether the speed, agility, aerobic and anaerobic capacities of football players varied by playing positions. Elite youth football players (n = 123, age = 15.7 ± 0.5 years) who played in six different positions, as goalkeepers (GK), full backs (FB), central defenders (CD), wide midfielders (WM), central midfielders (CM), and attackers (AT), were assessed. Multivariate analysis of variances was used to compare the following variables: Linear running sprint for 5 m (S5) and 10 m (S10), flying sprint for 20 m (F20), agility 505 test with turn on the dominant (A505D) and non-dominant leg (A505N), agility K-test, Yo-Yo intermittent recovery (YYIR1) test and repeat sprint ability (RSA) test. The results showed significant influence of playing positions on linear-running sprint performance (F1,123 = 6.19, p < 0.01, ηp² = 0.23). Midfielders reached significantly higher performance levels (CM = 2.44 ± 0.08 s, WM = 2.47 ± 0.13 s) in the A505N test compared to GK (2.61 ± 0.23 s). Outfield players had significantly higher performance in both YYIR1 and RSA tests compared to GK (p < 0.01). The results of this study may provide insightful strategies for coaches and clinical practitioners for developing position-specific conditioning programs.

#6 Energy expenditure and dietary intake in professional football players in the Dutch Premier League: Implications for nutritional counselling
Reference: J Sports Sci. 2019 Feb 16:1-9. doi: 10.1080/02640414.2019.1576256. [Epub ahead of print]
Authors: Brinkmans NYJ, Iedema N, Plasqui G, Wouters L, Saris WHM, van Loon LJC, van Dijk JW
Summary: Selecting effective dietary strategies for professional football players requires comprehensive information on their energy expenditure (EE) and dietary intake. This observational study aimed to assess EE and dietary intake over a 14-day period in a representative group (n = 41) of professional football players playing in the Dutch Premier League (Eredivisie). Daily EE, as assessed by doubly labelled water, was 13.8 ± 1.5 MJ/day, representing a physical activity level (PAL) of 1.75 ± 0.13. Weighted mean energy intake (EI), as assessed by three face-to-face 24-h recalls, was 11.1 ± 2.9 MJ/day, indicating 18 ± 15% underreporting of EI. Daily EI was higher on match days (13.1 ± 4.1 MJ) compared with training (11.1 ± 3.4 MJ; P < 0.01) and rest days (10.5 ± 3.1 MJ; P < 0.001). Daily carbohydrate intake was significantly higher during match days (5.1 ± 1.7 g/kg body mass (BM)) compared with training (3.9 ± 1.5 g/kg BM; P < 0.001) and rest days (3.7 ± 1.4 g/kg BM; P < 0.001). Weighted mean protein intake was 1.7 ± 0.5 g/kg BM. Daytime distribution of protein intake was skewed, with lowest intakes at breakfast and highest at dinner. In conclusion, daily EE and PAL of professional football players are modest. Daily carbohydrate intake should be increased to maximize performance and recovery. Daily protein intake seems more than adequate, but could be distributed more evenly throughout the day.

#7 Listening to neutral or self-selected motivational music during warm-up to improve short-term maximal performance in soccer players: Effect of time of day
Reference: Physiol Behav. 2019 Feb 25. pii: S0031-9384(18)30924-7. doi: 10.1016/j.physbeh.2019.02.033. [Epub ahead of print]
Authors: Belkhir Y, Rekik G, Chtourou H, Souissi N
Summary: The present experiment examined the effects of listening to different types of music during warm-up on the diurnal variation of short-term maximal performance (STMP) in soccer players, using a 3 × 2 mixed design with factors "Condition" (warm-up with self-selected motivational-music (WUMM) vs. warm-up with neutral-music (WUNM) vs. warm-up without-music (WUWM) and "Time of Day" (07 h00 vs. 17 h00). In a random order, twelve male soccer players performed a 5-m shuttle run test after a 10 min of WUMM, a 10 min of WUNM and a 10 min WUWM at 07 h00 and 17 h00. The higher distance (HD) and total distance (TD) were measured during the test, and the rated perceived exertion (RPE) and the feelings states (FS) were obtained immediately after the warm-up and the test. The results revealed that HD and TD were higher at 17 h00 than 07 h00 in all conditions (p < .01). At 07 h00 and 17 h00, TD and HD were higher after WUMM and WUNM than WUWM and after WUMM than WUNM (p < .01). This improvement was greater at 07 h00 than 17 h00 (e.g., 6.97% vs. 5.26% for TD). Moreover, FS were more positive after WUNM than WUWM only at 07 h00, after WUMM than WUWM at the two time-of-day (p < .01), and after WUMM than WUNM at 17 h00 (p < .01). After the 5-m shuttle run test, FS were more negative and the RPE scores were higher with WUMM than WUWM at 07 h00 (p < .01). The findings suggested that STMP and feelings depend on types of music listened during a warm-up. A warm-up with self-selected motivational-music improves STMP and feelings at 07 h00 and 17 h00 with greater enhancement in the morning. However, a warm-up with neutral-music improves STMP and feelings only at 07 h00.

#8 Effects of a Soccer Tournament on the Psychohormonal States of Collegiate Female Players
Reference: J Strength Cond Res. 2019 Feb 18. doi: 10.1519/JSC.0000000000002993. [Epub ahead of print]
Authors: Broodryk A, Pienaar C, Edwards D, Sparks M
Summary: A gap exists in the literature concerning the connection between soccer players' hormonal and psychological responses when playing a tournament, or even a match, and its outcome (victory or defeat). This study evaluates the effects of a week-long tournament on the psychohormonal states of collegiate female soccer players. Eight players' cortisol (saliva sample), mood states (Incredibly Short Profile of Mood States [ISP]), and state-anxiety (state subscale of the State-Trait Anxiety Inventory) were assessed 1 hour before and 15 minutes after every game. Cortisol increased significantly after most matches, with intramatch differences observed (p < 0.05, d > 1.2). Match intensity influenced cortisol secretion, with greater secretion as intensity increased. The ISP demonstrated intramatch differences for the subscales' fatigue, depression, tension, and vigor (p < 0.05). Matches lost produced a higher total mood disturbance (TMD) index compared with matches won (p = 0.001, d = 1.4). Cortisol correlated with the TMD and various mood subscales before a winning outcome, with the ISP correlating at all times with the anxiety scores (p < 0.05). In conclusion, these results indicate that physiological and psychological variables combine to contribute to the stress response during a tournament. Focusing on high-intensity activities and minimizing fatigue are important, as both are associated with raised cortisol and negative mood states. Finally, implementing a mood questionnaire over a tournament can be beneficial, as sensitive information on players' hormonal and perceived anxiety states, which subsequently affect physical performance, can be obtained.

#8 Concurrent changes in eccentric hamstring strength and knee joint kinematics induced by soccer-specific fatigue
Reference: Phys Ther Sport. 2019 Feb 18;37:21-26. doi: 10.1016/j.ptsp.2019.02.003. [Epub ahead of print]
Author: Greig M
Summary: The purpose was to investigate the influence of soccer-specific fatigue on concurrent changes in knee joint kinematics and hamstring strength, given the increased risk of injury during the latter stages of match-play and the prevalence of knee joint and hamstring muscular injury. Ten male professional soccer players participated in this study. Reactive inversion, eversion and neutral hop tasks were completed at 15 min intervals during a soccer-specific protocol, with touchdown knee joint kinematics in the frontal and sagittal planes calculated at 200 Hz. In a separate trial, players completed maximal eccentric knee flexions at 160°·s-1 (reflecting average knee angular velocity in the functional task) at 15 min intervals, quantifying peak torque. All trials were characterized by knee varus at touchdown, with ∼4° greater mal-alignment elicited over the final 15 min of the protocol (P ≤ 0.05). Peak eccentric hamstring strength was significantly (P = 0.045) reduced throughout the 2nd half. The coincident impairment of eccentric hamstring strength and increased knee varus at touchdown predisposes the player to injury, supporting epidemiological observations. Knee varus in these elite male players is in marked contrast to the valgus associated with ACL injury risk in female players.

#9 Evaluation of an In-Ear Sensor for Quantifying Head Impacts in Youth Soccer
Reference: Am J Sports Med. 2019 Feb 25:363546519826953. doi: 10.1177/0363546519826953. [Epub ahead of print]
Authors: Sandmo SB, McIntosh AS, Andersen TE, Koerte IK, Bahr R
Summary: Wearable sensor systems have the potential to quantify head kinematic responses of head impacts in soccer. However, on-field use of sensors (eg, accelerometers) remains challenging, owing to poor coupling to the head and difficulties discriminating low-severity direct head impacts from inertial loading of the head from human movements, such as jumping and landing. The purpose of the study was to test the validity of an in-ear sensor for quantifying head impacts in youth soccer. First, the sensor was mounted to a Hybrid III headform and impacted with a linear impactor or a soccer ball. Peak linear acceleration (PLA), peak rotational acceleration (PRA), and peak rotational velocity (PRV) were obtained from both systems; random and systematic errors were calculated with Hybrid III as reference. Then, 6 youth soccer players wore sensors and performed a structured training protocol, including heading and nonheading exercises; they also completed 2 regular soccer sessions. For each accelerative event recorded, PLA, PRA, and PRV outputs were compared with video recordings. Receiver operating characteristic curves were used to determine the sensor's discriminatory capacity in both on-field settings, establishing cutoff values for predicting outcomes. For the laboratory tests, the random error was 11% for PLA, 20% for PRA, and 5% for PRV; the systematic error was 11%, 19%, and 5%, respectively. For the structured training protocol, heading events resulted in higher absolute values (PLA = 15.6 g± 11.8 g) than nonheading events (PLA = 4.6 g± 1.2 g); the area under the curve was 0.98 for PLA. In regular training sessions, the area under the curve was >0.99 for PLA. A 9 g cutoff value yielded a positive predictive value of 100% in the structured training protocol versus 65% in the regular soccer sessions. The in-ear sensor displayed considerable random error and substantially overestimated head impact exposure. Despite the sensor's excellent on-field accuracy for discriminating headings from other accelerative events in youth soccer, absolute values must be interpreted with caution, and there is a need for secondary means of verification (eg, video analysis) in real-life settings. Wearable sensor systems can potentially provide valuable insights into head impact exposures in contact sports, but their limitations require careful consideration.

#10 Workload and injury incidence in elite football academy players
Reference: J Sports Sci. 2019 Mar 1:1-6. doi: 10.1080/02640414.2019.1584954. [Epub ahead of print]
Authors: Delecroix B, Delaval B, Dawson B, Berthoin S, Dupont G
Summary: The aim of this study was to prospectively analyse the relationship between workloads and injury in elite football academy players. Elite football academy players (n = 122) from under-19 (U19) and under-21 (U21) of a professional football team competing in UEFA European Cups were followed during 5 seasons. Injuries were collected and absolute workload and workload ratios (4-weeks, 3-weeks, 2-weeks and week-to-week) calculated using a rolling days method with the help of the session Rate of Perceived Exertion. There was no association between absolute workload or workload ratio with the injury incidence in the U19. In the U21, the level of cumulative absolute workloads during 3-weeks (RR = 1.39, p = 0.026) and during 4-weeks (RR = 1.40, p = 0.019) were associated with an increase in injury. There was no association between workload ratio and injury in U21. The significant link between high cumulated 3-weeks and 4 weeks workloads and injury in U21 confirmed the requirement to monitor the internal subjective workload in U21 in order to prevent injury. Further studies exploring the relationships between workload and injury are required in football academy.

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